Interpretive Summary: The number of studies on reovirus in turkeys is limited. These studies have described the role of avian reovirus in diseases such as arthritis, enteric disease syndromes, and immune system suppression. The enteric disease syndromes of young turkeys result in great economic loss to the industry. These enteric syndromes are not well understood, but several avian viruses have been detected and isolated from affected birds, including rotavirus, coronavirus, astrovirus and reovirus. Bacteria and protozoa are also associated with these enteric syndromes. The diagnosis of enteric disease in young turkeys is a common problem in California, second only to the diagnosis of colibacillosis--an infection caused by the Escherichia coli bacterium--in relative frequency among the diseases diagnosed between 1989 and 2001. Enteric disease in young turkeys is characterized by anorexia, diarrhea, dehydration, weight loss, high morbidity, and mortality. Turkey-origin reovirus is species-specific and can induce enteric disease and immunosuppression in turkeys but not in chickens. Recently, myocarditis (an inflammation of the heart muscle) associated with turkey reovirus was noted in young turkeys in California. This paper documents the clinical signs and pathology of myocarditis associated with reovirus in young turkeys based on reovirus isolation, detection using molecular diagnostics, nucleic acid sequencing, and microscopic examination of infected tissue.

Technical Abstract:
Myocarditis associated with reovirus was diagnosed in 17 day-old male turkey poults based on virus isolation, reverse transcript – polymerase chain reaction (RT-PCR), demonstration of reovirus antigen in the cytoplasm of mononuclear inflammatory cells and myocytes in the heart by immunohistochemistry (IHC) and reovirus particles in the endoplasmic reticulum of myocytes by transmission electron microscopy (TEM). Clinical signs in the poults included anorexia, growth depression and increased mortality. Gross lesions in the six poults examined were increased pericardial fluid, mild to moderate dilation of right ventricles and pale yellow myocardium and ascites. Other lesions in a few birds included mild pulmonary edema and congestion and pale serosa of the small intestine that had watery contents in their lumens. Microscopically in the heart there was mild to severe necrosis of myocytes and infiltration of primarily lymphocytes mixed with a few heterophils, macrophages and occasionally plasma cells and multinucleated giant cells. There was mild to moderate lymphoid depletion in the bursa of Fabricius. Reovirus was isolated from the heart of the turkey poults in chicken embryo liver cells and was confirmed by RT-PCR, IHC and TEM. A retrospective search of laboratory data base for cases of myocarditis associated with reovirus in turkeys revealed that this condition has occurred sporadically in California turkey flocks since 1991. This is the first documentation of myocarditis in turkey poults associated with reovirus.